Embedded memory, such as embedded dynamic random access memory (DRAM), is one of the fastest growing segments of the semiconductor industry. Two types of embedded DRAM processes currently exist: one that makes compact DRAM cells and low performance logic, and another that makes large DRAM cells and high performance logic. Embedded static random access memory (SRAM) processes also offer only compact SRAM cells with low performance logic or large SRAM cells with high performance logic. Thus, it is desirable to provide a process for manufacturing both compact embedded memory, such as compact DRAM or SRAM cells, and high performance logic, on the same chip.
In particular, in certain advanced DRAM processes, the memory gate stack has a nitride film on top, which allows a borderless contact to be made to the gate in a memory cell. On the other hand, certain high performance logic processes do not provide such a thick nitride film on top of the logic gate stack. The reason for this configuration is that a tall polysilicon-nitride stack would compromise across-chip linewidth variation (ACLV), which is a key parameter in maintaining the high performance desired in high performance logic.
Also, in many processes for combined logic and memory, the polysilicon gates in both regions are created simultaneously, as are the sidewall oxides. Because the optimum characteristics of gate and memory sidewall oxides are mutually exclusive (thin logic sidewall oxides and thick memory sidewall oxides are optimal), the sidewall oxides created simultaneously tend to reflect a compromise in characteristics which is not optimal for either region. In addition, logic well implants tend to be created at the same time as memory well implants, meaning that the logic well implants are subject to degradation during memory processing. Therefore, the combination of memory and logic processes has not resulted in optimal structural characteristics for either the memory or logic regions.
The deficiencies of the conventional semiconductor chip manufacturing processes show that a need still exists for a combined memory and logic creation process that provides the structural characteristics typically provided by stand-alone high performance logic processes and stand-alone compact embedded memory processes. To overcome the shortcomings of the conventional processes, a new process is provided. An object of the present invention is to provide a process that is compatible both with an advanced DRAM process that creates memory cells with nitride films on top, allowing a borderless contact between the gate and memory cell, and with a high performance logic process that creates a logic device without such a nitride film on top.
Another object of the present invention is to provide a process that forms the memory sidewall oxide as a step completely decoupled from the formation of the logic sidewall oxide. Thus, the memory sidewall oxide may be tailored for improved memory retention characteristics whereas the logic sidewall oxide may be tailored for improved logic device performance. Still another object of the present invention is to provide a process that completes the entire set of memory processing steps before the logic well implants are created. A related object is to prevent any substantial degradation in the logic device due to exposure to high temperature memory processing steps. It is yet another object of the present invention to provide a process in which the BPSG layer is deposited before logic gate formation. A related object is to permit densification of the BPSG layer at high temperature (thus allowing a tight-pitch memory array) without adversely affecting the logic devices.